This invention relates to industrial processes which call for the heating of coal, or the heating and drying of coal, to temperatures above 100.degree. C. or 212.degree. F. More particularly, this invention relates to a method for maintaining heating equipment for coal in standby operating condition, at a desired operating temperature in the heating equipment.
As an example, this invention has been adapted for use in a system for the charging of preheated coal into a coke oven wherein granular coal is preheated and conveyed as particles to coke ovens for the converstion of the coal particles into coke. While the preheated coal may be conveyed to the coke ovens by means of a coal charging car or larry car, preferable it is conveyed through a pipeline ("Pipeline Charging System"). In this example, the heating equipment for the coal is known as a "coal preheater" or as "preheating equipment" and patents which relate generally to the preheating of coal and charging the preheated coal into coke ovens include U.S. Pat. Nos.: 3,512,723 to J. A. Geoffroy; and 3,523,065 and 3,457,141, both to L. D. Schmidt.
In a Pipeline Charging System, large quantities of heat are supplied to the preheating equipment for the purpose of heating the coal to a temperature within the range of from 250.degree. to 700.degree. F. so that the coal is dry and below the temperature at which the coal is in a plastic state, thereby producing preheated coal prior to conveying the preheated coal particles through the pipeline. It is desirable to provide a method for maintaining the temperature of the various parts of the preheating equipment within a desired temperature range at times when the supply of coal being fed to the preheating equipment is either beginning, as during start-up of the equipment, or is interrupted.
The supply of coal being fed to the preheating equipment during start-up is normally gradually fed into the preheater until the feed rate reaches the desired rate while the equipment is maintained within the desired preheating range. However, the feed may be interrupted for any one of a number of reasons during operation of a coke plant. For example, any plant breakdown that delays the removal of the coke from the coke ovens may require interruption of the coal being fed to the preheating equipment. A malfunction in the preheating equipment or pipeline charging equipment may require interruption of the coal being supplied to the heating equipment. The normal effects of changes in personnel between the various shifts in the 24 hour operation of a coke plant may require interruption of the coal supply.
It is desirable to avoid the necessity of stopping the supply of heat to the preheating equipment or of greatly reducing the supply of heat, which would cool the preheating equipment to a temperature below the normal range of operating temperatures of the heating equipment and probably cause the oxygen content in the atmosphere in the heating equipment to rise. Such cooling is undesirable because when operation is resumed, the preheating equipment will then have to be brought up to the desired temperature again and down to the desired oxygen content. Instead, it is desirable to have a method of maintaining the heating equipment at the desired temperature during start-up operations or interruptions of coal feed and maintaining the heat source in a standby operating condition which keeps it in its normal range of operation with reference to heat output and combustion characteristics during below-capacity feed rates or interruptions of coal feed.
An emergency system is already known and used for preventing the temperature of the preheating equipment from rising to a dangerously high level during such interruptions of the coal feed in order to avoid damage to the equipment and possible formation of explosive mixtures. This prior art emergency system includes the injection of water automatically into the preheating equipment to absorb the heat. However, this emergency system causes substantially complete shutdown of the preheating equipment and does not maintain the heat output from a heat source in the heating equipment at a heat output level during interruption of coal feed which is within its normal range of heat output when coal feed is being supplied. Thus, this emergency system does not provide any method by which the temperature in the preheating equipment may be controlled in order to maintain the preheating equipment in standby operating condition, that is, at the desired level of temperature, during interruption of the coal feed.